CYA for Salt Chlorinator Pools II

[chem geek]

Richard & Poolsean,
The ONE thing that hasn't been addressed in your analysis, Richard, is the fact (using the term loosely) that almost everyone using a SWG with fairly well balanced water reports CC at 0 all the time. This is not true (I believe) of those that use bleach to maintain normal FC, at some point they will get a buildup of CC and have to shock (which is another interesting thread you are involved in!). This would seem to support the idea that the SWG is "super-shocking" in the cell and it must be some larger percentage of the water that passes through the cell than you are calculating.

In thinking about this though, you did say that it might take a couple of days to "shock" all of the water in the pool, perhaps this is happening and is "good enough" to keep the CC reading at or close to zero. I am wondering why or whether this information is available in Austrailia where they have been using these systems for a lot longer than here (at least for non-commercial pools).

A properly maintained outdoor pool using manually dosed chlorine will also report a CC of 0 all the time. If the chlorine level gets too low, then [EDIT]when you later add some more chlorine[END-EDIT] you can build up chloramines, but if it is constantly maintained, then this is much less likely [EDIT]since breakpoint occurs "on the fly" when sufficient chlorine is present[END-EDIT]. Also, outdoor pools exposed to sunlight may have UV breakdown the chloramines [EDIT]and possibly chlorinated organics[END-EDIT]. If you think about it, if you were to manually drip liquid chlorine (remember this is 60,000 ppm chlorine concentration) into your pool near an output jet and did so more or less continuously whenever the pump was running, you are essesntially doing exactly the same thing that the SWG system is doing [EDIT]including exposing some volume of water to very high chlorine levels[END-EDIT]. This implies that for a manually dosed pool it *may* be better to add chlorine in small doses more frequently rather than wait to do it in larger doses every few days [EDIT](and maybe this slower dosing helps keep the CC at 0 by oxidizing CC albeit a small amount of pool water volume at a time).[END-EDIT]

[EDIT]From what I have read, chlorinated organics (aka Disinfection ByProducts or DBPs) are more likely to form at higher chlorine concentrations (which CYA keeps down low) and in particular they form even more when there are chloramines also present so an incomplete breakpoint is the worst situation to be in (i.e. to not have enough chlorine to meet bather loads). The "bad" chlorinated organics, known as THMs, form more at higher pH while at lower pH you get more non-volatile chlorinted organics that are not considered harmful to health, but would still show up in CC if they persisted.[END-EDIT]

To really test your theory, an analysis of indoor pools with and without SWG would be the place to start. Indoor pools have a much harder time avoiding CC since there is no sunlight to break them down nor is there good air circulation to sweep away the products of breakpoint chlorination (though I've never really been convinced that this is an issue since nitrogen and oxygen gasses are already pretty dominant in air so I doubt they get that much higher in concentration above an indoor pool's surface -- perhaps it's other gasses, such as carbon dioxide, that need to be swept away???).

Richard

[chem geek]

Evan (waterbear) also makes good points. If we really want to get to the bottom of this, we would need ALL of the relevant pool water chemistry information that all of you in these forums know is what is asked of you when you want people to help you with your water problems. Of particular relevance for this SWG algae discussion would be minimum FC level, CYA, and pH. The TDS is already roughly known and not a big factor nor is the CH or alkalinity (except that very low alkalinity can lead to large pH swings you might not be aware of) -- I'm not talking about water balance to protect plaster, but rather those components that affect disinfecting chlorine concentration.

Anyway, I don't want to bash SWG systems in any way. Even though I don't have one, I think they are great and the most important service they provide is that they automatically maintain FC levels. And then there's that silky feel of salt water...oooooooo, you've got me thinking about getting an SWG myself!

[waterbear]

Anyway, I don't want to bash SWG systems in any way. Even though I don't have one, I think they are great and the most important service they provide is that they automatically maintain FC levels. And then there's that silky feel of salt water...oooooooo, you've got me thinking about getting an SWG myself!

IF you do you won't reget it! Also, you might be able to clear up some of the issues raised here!

Seriously, IMHO, SWGs are the way to go! Pool maintenance becomes much easier (I only have to worry about keeping a jug or two of acid on hand. Every time I buy bleach for the pool 'just in case' it gets used in the laundry since I don't need it for the pool! I keep an extra bag or two of salt on hand for the water softener and If I need to bump up the salt in the pool a bit I use some of that unless I decide to spluge on bag of Morten or Diamond Crystal pool salt...learned my lesson about off brand pool salt with I got iron stains from one!)

[mas985]

There are a few things that I would be interested in understanding better. I have pretty much forgotten everything learned in Chem 101 & 102.

From what I understand about SWGs, after the chlorine gas is generated, it dissolves very quickly into water and creates HOCL and HCL.

Based upon the amps in the cell, how much chlorine gas is generated and does this match the advertised production of the cell (see below)?

Does some of the HOCL and HCL revert back to salt before exiting the cell, which might explain the higher levels in the cell?

How long does it take for some of the HOCL to break down to OCL- & H+?

How long does it take CYA to bind to the chlorine?


Next, I found this derivation on a university web site:

In an electrolysis of sodium chloride solution experiment a current of 7 A was passed for 1 minute

o Electrode equations:
(-) cathode 2H+ + 2e- ==> H2 and (+) anode 2Cl- -2e- ==> Cl2
o (a) Calculate the volume of chlorine gas produced.
Q = I x t, so Q = 7 x 1 x 60 = 420 C
420 C = 420 / 96500 = 0.00435 mol electrons
this will produce 0.00435 / 2 = 0.002176 mol Cl2 (two electrons/molecule)
vol = mol x molar volume = 0.002176 x 24000 = 52.23 cm3 of Cl2
o (b) What volume of hydrogen would be formed?
52.23 cm3 of H2 because two electrons transferred per molecule, same as chlorine.


First, is this formulation correct?

If so, a cell running for 24 hours would produce 4418658 cm3, 4419 liters or 14 kg (31 lbs). This is 21x the advertised production rate so if true, a majority of chlorine produced quickly reverts back to salt before returning to the pool.

Anyway, I have measured the water coming out of the return and, as Sean pointed out, it was only 2-3 ppm higher than the pool water. So my premise is that there may be some HOCL in very high concentrations in the cell but does not survive very long.

[chem geek]

Sean knows his cells better than I, but I'll take a stab at explaining things from a general chemistry perspective and I'll do so in-line in bold in your quoted post below.

From what I understand about SWGs, after the chlorine gas is generated, it dissolves very quickly into water and creates HOCL and HCL.
True.

Based upon the amps in the cell, how much chlorine gas is generated and does this match the advertised production of the cell (see below)?
I do not know, but suspect the production is close to the generation except for the "side-reactions" that can occur and inefficiency from the buildup of products (Cl2 and HOCl).

Does some of the HOCL and HCL revert back to salt before exiting the cell, which might explain the higher levels in the cell?
There is always an equilibrium that goes on, though the generation is very fast and things are out-of-equilibrium at various points in the system. First chlorine gas is produced. Then this gets converted to HOCl. Then this gets bound up with CYA. So there *is* a peak of concentration of HOCl in some region of the water, but this exact same thing also happens (minus the chlorine gas) when you add liquid chlorine or bleach to your pool.

How long does it take for some of the HOCL to break down to OCL- & H+?
This is an extraordinarily fast reaction, but I don't have the rate constant at hand (in general, protonation reactions with acids are extraodinarily fast).

How long does it take CYA to bind to the chlorine?
Now in this case I have the actual rate constant and the answer depends on the concentration of the different CYA species in the water. The reaction of CYAs taking up HOCl in pool water is as follows at 80 ppm CYA:

... rate ......... [CYA-]
7.4x10^4 * 4.64x10(-4) * [HOCl] = 34 * [HOCl]
... rate ......... [CYA(2-)]
2.2x10^7 * 7.98x10^(-8) * [HOCl] = 1.8 * [HOCl]

Though the reaction rate is dependent on the HOCl concentration, one can calculate the half-life of HOCl which is how long it takes for half of the HOCl to get converted as follows (I'll only use the faster of the two reactions which is the CYA- one):

Half-life of HOCl = -ln(0.5)/34 = 0.02 seconds

It is this rather slow reaction (in chemistry 0.02 seconds seems like an eternity) that is why such high CYA levels are required to make the salt cell more efficient. If CYA reacted faster, then lower levels would "take up" the HOCl fast enough to keep the cell operating efficiently. In fact, if you calculate how long water spends inside the salt cell at normal flow rates, you get a time that is only somewhat longer than the 0.02 seconds which is why the 80 ppm CYA concentration is needed (for today's sizes of cells and chlorine production rates). With a CYA of 20 ppm, the time rises to around 0.1 seconds which is longer than the time water spends inside the salt cell. This means that Cl2 and HOCl build up more inside the cell and that degrades the efficiency of the cell.


Next, I found this derivation on a university web site:

In an electrolysis of sodium chloride solution experiment a current of 7 A was passed for 1 minute

o Electrode equations:
(-) cathode 2H+ + 2e- ==> H2 and (+) anode 2Cl- -2e- ==> Cl2
o (a) Calculate the volume of chlorine gas produced.
Q = I x t, so Q = 7 x 1 x 60 = 420 C
420 C = 420 / 96500 = 0.00435 mol electrons
this will produce 0.00435 / 2 = 0.002176 mol Cl2 (two electrons/molecule)
vol = mol x molar volume = 0.002176 x 24000 = 52.23 cm3 of Cl2
o (b) What volume of hydrogen would be formed?
52.23 cm3 of H2 because two electrons transferred per molecule, same as chlorine.


First, is this formulation correct?
Yes, this computation is correct (some constants were rounded so the final answer isn't as accurate as implied, but its essentially correct). The 0.002176 moles of chlorine is equivalent to (0.002176 moles/minute)*(2*35.4532 g/mole)/(453.59237 grams/pound)*60(min/hr)*24(hours/day) = 0.49 (pounds/24hours)

If so, a cell running for 24 hours would produce 4418658 cm3, 4419 liters or 14 kg (31 lbs). This is 21x the advertised production rate so if true, a majority of chlorine produced quickly reverts back to salt before returning to the pool.


52.23 * 60 * 24 = 75211 cm3, not 4418658 that you calculated. It looks like you multiplied by an extra factor of 60 thinking that the above example was per second when it was done for one minute.

The electrolysis current was 7 Amps. One of the manufacturers I looked up said their cell current was 4.5 to 7.8 amps, presumably adjustable as a "power level". They claim they can produce up to 1.45 pounds per day of chlorine. Remember that we calculated 0.49 pounds per day of Cl2 above. I don't know how to reconcile that, but the cell certainly isn't producing more chlorine than they claim.


Anyway, I have measured the water coming out of the return and, as Sean pointed out, it was only 2-3 ppm higher than the pool water. So my premise is that there may be some HOCL in very high concentrations in the cell but does not survive very long.
There is most certainly some HOCl in very high concentrations in the cell and most of it gets bound up by CYA, but that is also what happens when you add liquid chlorine to the pool.

If instead there were huge amounts of HOCl produced over larger areas of the cell and if this did in fact break down back into chloride ion before getting into the pool, then the "super-chlorination" effect would be real. However, over the short lifetime of the water from the cell and into your pool, the water can tolerate incredibly high concentrations of HOCl without breaking down. Remember that 12.5% chlorine (which is around 11% available chlorine) is over 100,000 ppm in concentration and yet takes months to breakdown (the half-life under normal conditions is over 200 days) naturally (at normal temperature and *not* exposed to sunlight).

Now if you had a special kind of salt cell that had TWO reactions in it where it generated HOCl at one end of the cell and then intentionally destroyed HOCl at the other end of the cell, then this would produce a middle area with high levels of HOCl that could super-chlorinate, but it is my understanding that this is not how the salt cells operate. Instead, they operate more like you described with the electrode equations.

[EDIT]
The fact is that the computations based on current (amps) shows that the salt cell simply does not produce enough chlorine to cause the average chlorine level of the entire cell to be higher than what you are measuring at the outlet which is just a few ppm higher UNLESS some water in the cell is moving much more slowly (more on that below). We have a conflict between the claimed 1.45 pounds/24hour maximum rate and the 0.5 pounds/24hour rate calculated from the amperage, but let's just see what we get using some reasonable numbers. One user in an earlier post estimated 90 GPM flow rates and I'd say that's about the same as my pool as well so let's use that.
0.5(pounds/24hour) * 453.952337(g/pound) * 1000(mg/g) / 24(hours/24hour) / 60(minutes/hour) = 158(mg/minute)
90(gallons/minute) * 3.785(liters/gallon) = 341(liters/minute)
158(mg/minute)/341(liters/minute) = 0.46 ppm
So the increase in chlorine at the outlet would only be 0.46 ppm at maximum. Since people measure more than this, something is wrong. If the 1.45 pounds/24hr is correct and the amperage is wrong, then you would get around 1.5 ppm incrementally higher output which sounds closer to being what people are measuring.

So how is it that someone can measure the ppm inside a cell and find it to be so high, seemingly over a reasonably large volume of the cell? Ignoring problems with how you perform such a measurement, if there are parts of the cell with slowly moving water then this water can indeed have very high chlorine concentration, but that means that most water moving through the cell bypasses this high chlorine area so even though it seems like a good-sized volume of the cell has high chlorine (which is true), it is not true that this "superchlorination" is getting applied to large volumes of pool water.

Sean mentioned how slowing down the water made it possible to give such water enough time to be exposed to the superchlorination so all of that makes sense, but it also means that the amount of total pool water exposed to this superchlorination is very small. So my point about taking many turnovers to get much of the water through this superchlorination is still valid. And this is essentially the same situation that happens when you add liquid chlorine (or bleach) to the pool except that an SWG dones this much more slowly over an extended period of time and therefore is more efficient and better. I just don't like the claim that implies a superchlorination of your entire pool water over a short period of time -- say, a day or two -- and leaving out the fact that bugs or algae stuck to walls aren't super-chlorinated at all.
[END-EDIT]

[brent.roberts]

I've onlhy been running the SWG for a few weeks now and have observed
that the pool liner is "squeaky clean" ... the exact opposite of slimy.

I have been raising the CYA since discovering it all went away during the winter.
It is now in the 40-50 range
Keep the ph in 7.2 to 7.5 range.
CC always at or near 0
FC 3 to 5... still playing with settings to get a balance between run time and power.

We have an unprotected hill top site, so lots of debris. Every morning about 30 to 50 crickets, equal numbers of some other beetle like bug, and 15 or so young frogs ... mostly dead by morning.

The water is astonishingly clear, despite the debris.

Reading all this I'm inclined to keep moving the CYA upward.

[chem geek]

Sean,

I updated the end of my post
See Above
to reflect what is consistent with your point of slower moving water in the cell getting longer contact time in a superchlorinated environment. We haven't heard from you so I hope I didn't offend. The theories about extra chlorine being generated and then breaking down just don't add up, but the theory of slower moving water at higher chlorine concentration does make sense. It's just that this doesn't actually superchlorinate all of the pool water quickly -- it would take weeks. You can either slow down the flow in a good part of the cell and have this part be exceptionally superchlorinated or you can have the water flow evenly through the entire cell at small chlorine boost, or a combination of the two, but you can't get the full benefits of both (superchlorination and full pool volume) at the same time.

Richard

[waterbear]

Richard,
I don't mean to offend you but it is obvious that you have never worked with or lived with a pool that has a SWG. Chlorine levels will change very slowly with changes in pump run time and cell output...sometimes it can take days for it to stabilize. While you are right that only the water passing through the plates is superchlorinated, in actual practice this seems to be enough to keep the CC at 0 with FC levels that are way below the best guess chart. Also, since algae will first start in the water column before it attaches to the sides and walls of the pool, once again, in actual practice, this seems to be sufficient to prevent algae blooms in normal operation. I view this in the same light as UV sanitizers and ozone generators. Any sanitation only takes place in the reaction chamber and not in the pool proper. The residual chlorine in the water takes care of the rest in all threee of these cases. Perhaps SWGs need to be looked at in the light of this as having 2 separate functions: To generate a residual FC level for the pool and to superchlorinate a small quantity of water in the reaction cell during cell operation to constantly destroy free floating algae and oxidize any nitrogenous compounds in much the same way an ozonator would. As far as superchlorinating all the water, the boost or superchlorinate functions on most units are, IMHO, not really good for this since it can take up to 24 hours to reach the high chlorine levels necessary. I feel it is much more effective to add sodium hypochlorite in the recommeded dosage all at once than to let it gradually climb up. If I understand breakpoint chlorination properly then many undsirable forms of combined chloramines would form at high chlorine levels that are below breakpoint and make breakpoint even more difficult to achieve thus the gradual rise in FC levels by running the celll 24/7 at 100% are not the optimum way to achieve it and may acutally hinder it.
I have only emperical evidence from my own experimentation with my pool but I can tell you this. If I maintain the cya in the 60-80 range and my FC at least 3 ppm and my pH 7.6 or below I do not have any CC nor does the pool ever get cloudy. I have never had an algae breakout since the SWG went online. I have experimented with the above factors and have found that
1. When my CYA is below 60 ppm I have to turn the output up to maintain a 3 ppm FC level.
2. When my pH is above 7.6 I start to show some CC on testing. When it reaches 8.0 my FC level starts to drop.
3. When my FC level is below 3 ppm I have experienced CC and cloudiness of the water.

It is a given that theory often does not hold up in field conditions because of other factors that are not taken into account.

Your thoughts on these points would be appreciated.

[chem geek]

Richard,
I don't mean to offend you
Perhaps SWGs need to be looked at in the light of this as having 2 separate functions: To generate a residual FC level for the pool and to superchlorinate a small quantity of water in the reaction cell during cell operation to constantly destroy free floating algae and oxidize any nitrogenous compounds in much the same way an ozonator would.
It is a given that theory often does not hold up in field condtions because of other factors that are not taken into account.
Your thoughts on these points would be appreciated.

Absolutely no offense taken at all. You are correct that I have zero experience with SWG systems, I agree completely with your points, and I'm the one who has to be careful not to offend. My educated guess as to what is going on is not inconsistent with the real-world results and yes, it's the theory that must match the real world, not the other way around. I was just dumbfounded with the initial claim that the salt cell has superchlorinated levels of chlorine AND that this was getting applied to all of the pool water (in a reasonable time).

There is one big difference between UV and ozone sanitizers vs. SWG as far as I understand them and that is that the former do in fact "do their work" on the entire volume of water that flows through their chambers so that after a few hours with one turnover of pool water, the majority (I forget the number and how to calculate it, but I think it's around 70%) of the water in the pool has been sterilized and oxidized at least once. This does not appear to be the case with the SWG and instead some fraction of the water goes through amounts of superchlorination. I could be dead-wrong about how the UV and ozone systems work so if anyone knows if they "slow down" some part of the water in their systems in order to more effectively disinfect and oxidize, please let us know.

Since putting in liquid chlorine (bleach) also superchlorinates a part of the pool water when it is introduced into the pool, then the incremental benefit of the SWG probably derives from its continual dosing which is more optimal. I wonder if people without SWG were to manually dose their pools more frequently (with smaller amounts of chlorine each time) and did so over pool jets at both ends of the pool if they, too, could operate at lower levels of chlorine safely. My guess is that they could, but I have no idea what the required FC level would be between "once a day" well-distributed manual dosing vs. the continual dosing of the SWG.

The 3 ppm FC level that you found to be required for your pool is still very safe from a disinfection point of view since 70 ppm CYA, 7.6 pH and 3 ppm FC gives 0.017 ppm HOCl which is above the 0.011 "minimum" that appears to be needed for disinfection (and that's in non-SWG pools and spas).

Again, I think SWG is great and just because I "bash" a specific claim doesn't mean I think the technology as a whole has no net benefit. On the contrary, I regret not having one installed when we put in our pool (it was not mentioned as an option by our pool contractor and I knew even less about pools then than I do now).

[waterbear]

Absolutely no offense taken at all. You are correct that I have zero experience with SWG systems, I agree completely with your points, and I'm the one who has to be careful not to offend. My educated guess as to what is going on is not inconsistent with the real-world results and yes, it's the theory that must match the real world, not the other way around. I was just dumbfounded with the initial claim that the salt cell has superchlorinated levels of chlorine AND that this was getting applied to all of the pool water (in a reasonable time).
You are right that it is only being applied to a fraction of the pool water but it is an ongoing process and I would believe the effect would become cumulative after a while...in much the same way UV sterilization would only kill the bacteria in the reaction tube but with each pass there would be less and less bacteria in the whole of the water. I know that UV light has to be in close proximity to the water for sterilization to occur from my experience with it in aquarium use and not all the water flowing through the tube receives enought intensity to completely kill the pathogens present. Ozone, also only works on the water in the reaction chamber. In fact, no residual ozone is supposed to be in the water when it enters the pool. Once again the benifit seems to be gained from an incremental effect. Plus ozone acutally depletes chlorine levels somewhat so a negative factor is introduced.
There is one big difference between UV and ozone sanitizers vs. SWG as far as I understand them and that is that the former do in fact "do their work" on the entire volume of water that flows through their chambers so that after a few hours with one turnover of pool water, the majority (I forget the number and how to calculate it, but I think it's around 70%) of the water in the pool has been sterilized and oxidized at least once. This does not appear to be the case with the SWG and instead some fraction of the water goes through amounts of superchlorination. I could be dead-wrong about how the UV and ozone systems work so if anyone knows if they "slow down" some part of the water in their systems in order to more effectively disinfect and oxidize, please let us know.

Since putting in liquid chlorine (bleach) also superchlorinates a part of the pool water when it is introduced into the pool, then the incremental benefit of the SWG probably derives from its continual dosing which is more optimal. I wonder if people without SWG were to manually dose their pools more frequently (with smaller amounts of chlorine each time) and did so over pool jets at both ends of the pool if they, too, could operate at lower levels of chlorine safely.
It would seem that systems that use ORP controllers and peristaltic pumps would achieve this but I don't believe that optimin ORP mv levels are reached with lower chlorine concentrations. And it is a well known fact that CYA disturbs ORP readings. Whether these actually have an effect on the actual sanitation I do not know. I would be intersting to compare pools with SWGs and peristaltic pumps at comparable FC levels and CYA levels and see if, in actual use, similar results would be obtained.
My guess is that they could, but I have no idea what the required FC level would be between "once a day" well-distributed manual dosing vs. the continual dosing of the SWG.

The 3 ppm FC level that you found to be required for your pool is still very safe from a disinfection point of view since 70 ppm CYA, 7.6 pH and 3 ppm FC gives 0.017 ppm HOCl which is above the 0.011 "minimum" that appears to be needed for disinfection (and that's in non-SWG pools and spas).

Again, I think SWG is great and just because I "bash" a specific claim doesn't mean I think the technology as a whole has no net benefit. On the contrary, I regret not having one installed when we put in our pool (it was not mentioned as an option by our pool contractor and I knew even less about pools then than I do now).

A most interesting discourse!